Multiple levels of regulation for self-tolerance in beef insulin transgenic mice

To characterize the mechanism(s) of tolerance toward soluble self-antigens (Ags), beef insulin (BI) transgenic (Tg) mice were generated in which the transgene was expressed in pancreatic beta-cells. Our previous data showed that: (i) Ag-specific tolerance can be induced and/or maintained in peripheral T cells in thymectomized BI-Tg mice and (ii) CD4+ Th2 regulatory T cells are involved in maintaining peripheral tolerance (by anti-BI antibody response). In this paper, we have further characterized the relationship of low levels of BI expression (10(-10)-10(-11) M) in Th1/Th2 activation. In addition, we have explored intrathymic events associated with tolerance to self-Ags not expressed in the thymus and/or to circulating self-Ags. Limiting dilution analysis showed that there was a significantly higher frequency of BI-specific Th2 cells in Tg mice with a corresponding higher frequency of Th1 cells in non-Tg mice. While there was no transgene expression in the thymus (by RT-PCR), independent studies showed that BI can be processed and presented in the Tg thymus, which correlated with the Ag-specific hyporesponsiveness of mature thymocyes detected in vitro. High-dose rIL-2 (150 U/ml) was able to restore in vitro peripheral T cell response of Tg mice to levels comparable to those of the non-Tg control. Collectively, our data suggest that: (i) there is a differential activation of BI-specific Th1/Th2 cells in vivo in the presence of low Ag concentration; (ii) the thymus may play a role in self-tolerance to Ags whose expression in adults is restricted to the periphery; and (iii) multiple levels of regulation such as thymic selection, peripheral anergy, and active suppression may be involved in tolerance to BI in BI-Tg mice.